The present invention relates to cable testing equipment for use in the telecommunications field.
There is a continuing need in the telecommunications and CATV industry for cable testing equipment that permits rapid and low-cost identification of physical cable conductor faults. In the typical telephone cable, there are groups of telephone pairs, each pair comprising a ring and tip conductor. These conductors are insulated from each other and from all other conductors in the cable. A typical telephone cable may vary in size from approximately 25 pairs to as many as 2700 pairs or more.
When a new telephone cable is installed, and before service can be connected, it is common in the industry to test all of the pairs in the cable to determine if there are any physical faults in the pairs. After such initial testing, the cable may be further tested for signal transmission characteristics. Because of the large number of conductors within each cable, it will be appreciated that test equipment which can rapidly examine and simply and quickly indicate the existence and type of fault in a particular pair can provide a considerable reduction in installation and maintenance costs.
While there are numerous types of faults which may exist due to defects in the manufacture of the cable or in the cable installation, typical cable pair faults are: (1) shorts -- the conductors of a given pair have an electrical contact between them; (2) grounds -- one conductor of one pair is in electrical contact with ground; (3) open-one-side -- one conductor of one pair lacks continuity to the end of the cable; (4) open-both-sides (tip and ring) -- both conductors of one pair lack continuity to the end of the cable; (5) split -- one conductor of one pair is improperly spliced with a conductor of a second pair (i.e., the tip, for example, of pair one is improperly connected to the tip or ring conductor of another pair); (6) cross -- one conductor of one pair is in electrical contact with a conductor of a different pair; and (7) punchbacks (sometimes called buttbacks) -- both conductors of one pair in one section of a cable have been spliced back to the conductors of another pair in the same section of cable rather than in the next adjacent cable section.
One common construction technique in the telephone industry for the installation of a new telephone cable is to commence cable laying at the central office. In the central office, the cable pairs are terminated at the main distribution frame. Successive sections are then spliced at increasingly remote locations from the central office. These splices are commonly made in underground manholes provided for such purpose.
The above described faults may appear in either the cable sections, due to defective manufacture or damage caused by "pulling in" of the section, or in improper splices between adjacent sections. To locate such faults, a testing procedure has been established. By this procedure, after several sections of the new cable have been placed, each successive section and the splices therebetween are tested for physical faults.
There is now known in the art a device that performs a rapid test of pairs during installation generally in the manner described above. This device is capable of identifying the existence of DC faults such as shorts and grounds. It is further possible to identify the existence of opens, splits, crosses and punchbacks utilizing the capacitance of the cable pair.
In operation, a plurality of cable pairs are connected to the test instrument from the main distribution frame through test connectors commonly referred to as "front top shoes." Such shoes will typically connect 50 pairs to the test instrument at once. Through the provision of a pair selector switch, anyone of these pairs may then be electrically connected to the test instrument for fault testing. Through the manipulation of various knobs and levers, different test circuits may be sequentially connected to the pair under test to determine if a fault exists and to identify the type of fault, if any. The existence of a fault is indicated by visual monitoring of a meter while a test sequence is performed.
Basically, the prior art apparatus operates to selectively monitor the distributed capacitance of each pair and to compare the results with all other pairs of conductors. To accomplish this, the apparatus may include a cable pair selector switch, a relay circuit, a timing circuit, a source of DC voltage (usually a battery for portability), and a meter circuit. The device may further be provided with a footage capacitance measuring circuit so that the location, as well as the identity, of certain types of faults such as opens, punchbacks and splits may be ascertained.
Although such devices operate satisfactorily for cables of considerable length where the distributed capacitance is large and readily measured, problems have been encountered when testing shorter cables. More particularly, as a pair capacitance decreases, i.e., the length of the cable becomes relatively small, the interaction between the cable capacitance and the testing circuit is not sufficiently pronounced to provide good, easily read test results. Whereas, the prior art device has functioned satisfactorily when at least ten thousand feet of cable is connected to the test set, where the cable has a lesser length, the device is increasingly ineffective. For example, at a length of 500 feet or less, it is not possible to make a satisfactory fault test.